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package com.wf.captcha.base;
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import java.util.Collections;
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import java.util.Stack;
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/**
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* 字符串计算器
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* @link https://www.cnblogs.com/woider/p/5331391.html
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*/
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public class Calculator {
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private final Stack<String> postfixStack = new Stack<String>();// 后缀式栈
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private final Stack<Character> opStack = new Stack<Character>();// 运算符栈
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private final int[] operatPriority = new int[]{0, 3, 2, 1, -1, 1, 0, 2};// 运用运算符ASCII码-40做索引的运算符优先级
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public static double conversion(String expression) {
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double result = 0;
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Calculator cal = new Calculator();
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try {
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expression = transform(expression);
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result = cal.calculate(expression);
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} catch (Exception e) {
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// e.printStackTrace();
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// 运算错误返回NaN
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return 0.0 / 0.0;
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}
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// return new String().valueOf(result);
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return result;
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}
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/**
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* 将表达式中负数的符号更改
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*
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* @param expression 例如-2+-1*(-3E-2)-(-1) 被转为 ~2+~1*(~3E~2)-(~1)
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* @return
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*/
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private static String transform(String expression) {
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char[] arr = expression.toCharArray();
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for (int i = 0; i < arr.length; i++) {
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if (arr[i] == '-') {
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if (i == 0) {
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arr[i] = '~';
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} else {
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char c = arr[i - 1];
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if (c == '+' || c == '-' || c == '*' || c == '/' || c == '(' || c == 'E' || c == 'e') {
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arr[i] = '~';
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}
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}
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}
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}
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if (arr[0] == '~' || arr[1] == '(') {
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arr[0] = '-';
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return "0" + new String(arr);
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} else {
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return new String(arr);
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}
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}
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/**
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* 按照给定的表达式计算
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*
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* @param expression 要计算的表达式例如:5+12*(3+5)/7
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* @return
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*/
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public double calculate(String expression) {
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Stack<String> resultStack = new Stack<String>();
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prepare(expression);
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Collections.reverse(postfixStack);// 将后缀式栈反转
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String firstValue, secondValue, currentValue;// 参与计算的第一个值,第二个值和算术运算符
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while (!postfixStack.isEmpty()) {
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currentValue = postfixStack.pop();
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if (!isOperator(currentValue.charAt(0))) {// 如果不是运算符则存入操作数栈中
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currentValue = currentValue.replace("~", "-");
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resultStack.push(currentValue);
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} else {// 如果是运算符则从操作数栈中取两个值和该数值一起参与运算
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secondValue = resultStack.pop();
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firstValue = resultStack.pop();
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// 将负数标记符改为负号
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firstValue = firstValue.replace("~", "-");
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secondValue = secondValue.replace("~", "-");
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String tempResult = calculate(firstValue, secondValue, currentValue.charAt(0));
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resultStack.push(tempResult);
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}
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}
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return Double.valueOf(resultStack.pop());
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}
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/**
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* 数据准备阶段将表达式转换成为后缀式栈
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*
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* @param expression
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*/
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private void prepare(String expression) {
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opStack.push(',');// 运算符放入栈底元素逗号,此符号优先级最低
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char[] arr = expression.toCharArray();
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int currentIndex = 0;// 当前字符的位置
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int count = 0;// 上次算术运算符到本次算术运算符的字符的长度便于或者之间的数值
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char currentOp, peekOp;// 当前操作符和栈顶操作符
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for (int i = 0; i < arr.length; i++) {
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currentOp = arr[i];
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if (isOperator(currentOp)) {// 如果当前字符是运算符
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if (count > 0) {
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postfixStack.push(new String(arr, currentIndex, count));// 取两个运算符之间的数字
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}
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peekOp = opStack.peek();
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if (currentOp == ')') {// 遇到反括号则将运算符栈中的元素移除到后缀式栈中直到遇到左括号
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while (opStack.peek() != '(') {
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postfixStack.push(String.valueOf(opStack.pop()));
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}
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opStack.pop();
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} else {
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while (currentOp != '(' && peekOp != ',' && compare(currentOp, peekOp)) {
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postfixStack.push(String.valueOf(opStack.pop()));
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peekOp = opStack.peek();
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}
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opStack.push(currentOp);
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}
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count = 0;
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currentIndex = i + 1;
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} else {
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count++;
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}
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}
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if (count > 1 || (count == 1 && !isOperator(arr[currentIndex]))) {// 最后一个字符不是括号或者其他运算符的则加入后缀式栈中
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postfixStack.push(new String(arr, currentIndex, count));
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}
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while (opStack.peek() != ',') {
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postfixStack.push(String.valueOf(opStack.pop()));// 将操作符栈中的剩余的元素添加到后缀式栈中
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}
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}
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/**
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* 判断是否为算术符号
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*
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* @param c
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* @return
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*/
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private boolean isOperator(char c) {
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return c == '+' || c == '-' || c == '*' || c == '/' || c == '(' || c == ')';
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}
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/**
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* 利用ASCII码-40做下标去算术符号优先级
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*
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* @param cur
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* @param peek
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* @return
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*/
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public boolean compare(char cur, char peek) {// 如果是peek优先级高于cur,返回true,默认都是peek优先级要低
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boolean result = false;
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if (operatPriority[(peek) - 40] >= operatPriority[(cur) - 40]) {
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result = true;
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}
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return result;
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}
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/**
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* 按照给定的算术运算符做计算
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*
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* @param firstValue
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* @param secondValue
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* @param currentOp
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* @return
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*/
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private String calculate(String firstValue, String secondValue, char currentOp) {
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String result = "";
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switch (currentOp) {
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case '+':
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result = String.valueOf(ArithHelper.add(firstValue, secondValue));
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break;
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case '-':
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result = String.valueOf(ArithHelper.sub(firstValue, secondValue));
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break;
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case '*':
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result = String.valueOf(ArithHelper.mul(firstValue, secondValue));
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break;
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case '/':
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result = String.valueOf(ArithHelper.div(firstValue, secondValue));
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break;
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}
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return result;
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}
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}
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